Many devices can be used to characterize items of currency. For example, a validation device, comprising a validation unit, can be used to characterize an item of currency. For the purposes of the disclosure, the term item of currency includes, but is not limited to, valuable papers, security documents, banknotes, checks, bills, certificates, credit cards, debit cards, money cards, gift cards, coupons, coins, tokens, and identification papers. In such state of the art devices the validation unit includes a sensing system often further comprising a source for emitting light and a receiver for receiving the emitted light. Validation (i.e., classification) of a currency item can involve the measurement and analysis of one or both of reflected light and light transmitted through a currency item.
Typical validation units are arranged to use a plurality of light emitting sources (e.g., Light Emitting Diodes (LEDs)) to gather reflective and/or transmission responses from a currency item. Generally these sources are configured such that they emit light within a relatively narrow band of wavelength within a spectrum. More particularly, commonly known sources (e.g., red LEDs, blue LEDs, and green LEDs) typically have an emission spectrum with a narrow band (between 15 nm and 35 nm). Examples of common sources can include red sources emitting light in the range of 640 nm to 700 nm, blue sources emitting light in the range of 450 nm to 480 nm, and green sources emitting light in the range of 520 nm to 555 nm. Often such common sources are configured to emit light within wavelength bands consistent with known colors within the visible spectrum (e.g., red light, blue light and green light). The response of a currency item to being illuminated with sources having emission within known color spectrums of visible light can be used to determine various characteristics about the item of currency. In some cases infrared light can be used to gather information about characteristics of an item of currency.
There exist image processing machines (e.g., document scanners or photocopiers) which use a plurality of sources and detectors to reproduce or store and image of a document. In the case of color images, it is often the goal of such image processing machines to gather characteristics from a document such that they can be reproduced to be visually equivalent to the human eye (i.e., discrimination like the human eye is capable of). The fact that the human eye acts like a three color imaging system, allows for the design of such image processing machines to be developed that reproduce a color image in a way that the human eye (or any imaging system with similar color limitations) cannot discriminate between the original image and the reproduced image.
A limitation of some current devices for classifying items of currency is that the typical common sources used result in gaps within the whole spectrum because each source generally emits in a narrow band of spectrum. One solution to this problem is to use a very large number of common type sources such that there would be sufficiently enough sources to cover the entire spectrum. This solution is undesirable because it leads to a very large and expensive validation apparatus. Furthermore, such a solution results in a device required to process very large amounts of data and thus is not as efficient as required for a currency validation apparatus (e.g., gaming machine, vending, machine, and ticketing machine, etc.) where validation is needed to be made in a relatively short period of time (e.g., less than one second).
State of the art devices can illuminate a currency item using sources within the validation unit either in a sequential manner (i.e., where each emitter illuminates in a different wavelength band) or simultaneously. Such a validation system is disclosed by U.S. Pat. No. 5,632,367, which is incorporated herein by reference in its entirety. Additionally, a validation unit can illuminate a currency item using a light bar type system to mix light from a plurality of sources. Such a light mixing system is disclosed in U.S. Pat. No. 6,994,203 and is incorporated herein by reference in its entirety.
A currency item being characterized by a validation unit can be discriminated in various ways commonly known in the art (e.g., Malahanobis Distance, Feature Vector Selection, or Support Vector Machine). Currency items can be characterized based on their color response as disclosed in currently pending U.S. Provisional Application Ser. No. 61/137,386, which is incorporated herein by reference.